With the rapid and wide dissemination of information media, such as portable electronic equipment and personal computers, semiconductor devices have been required to have larger capacities, faster processing speeds, and smaller sizes. To meet these requirements, there is an urgent need for facilities or technologies with or by which high-density semiconductors that are improved in degree of integration, reliability, and data accessibility can be fabricated.
A capacitor, which is one of semiconductor device elements, is structured to have a storage node for a lower electrode, and a plate node for an upper electrode, with a dielectric film intercalated therebetween. For a high degree of integration of semiconductor devices, a high-capacitance capacitor that guarantees sufficient electrostatic capacity is needed.
In a capacitor, the capacitance is directly proportional to both the surface area of the electrodes and the dielectric constant of the dielectric film and inversely proportional to the thickness of the dielectric film. Accordingly, various methods for ensuring sufficient capacitance have been directed towards the development of a dielectric membrane with a large dielectric constant, the reduction of the thickness of a dielectric film, the enlargement of the surface area of capacitor storage electrodes, and the reduction of the distance between electrodes, etc.
For example, capacitors with a 3-dimensional structure, such as concave-type or cylinder-type capacitors, were developed to enlarge the surface area of storage electrodes. In recent years, rather than a concave-type capacitor that utilizes only the internal area as the node area, a cylinder-type capacitor that utilizes the external area as well as the internal area as the node area has been practically applied in the mass-production of semiconductor devices.
Typically, the fabrication of a 3D cylinder-type capacitor starts with the formation of a capacitor pattern, followed by dry etching the capacitor oxide film to form a trench within which a storage electrode is constructed. Then, the capacitor oxide film is removed by a wet-dip process using a wet chemical including HF or NH4F.
In the past, no problems were found in this wet etching process when the aspect ratio of the capacitor was low. However, as aspect ratios have increased, the capacitor is apt to collapse when finally washed with distilled water alone.
The force of causing the collapse of the patterns formed on the substrate is in proportion to the surface tension of the liquid used in the pattern formation and washing process and the cosine value of the contact angle of the liquid on against the patterns. Hence, the reduction of the liquid surface tension has arisen as an issue and a solution to the problem of collapse is being actively researched. In practice, the following methods are employed.
To prevent the patterns from collapsing, the manufacturers either use an alcohol such as isopropanol or a surfactant to reduce the surface tension on the patterns, or increase the supporting force of the patterns per se by connecting the patterns to a support such as bridge or by tying the patters in a bundle.
Recently, the fabrication of semiconductor devices is being achieved by finer lithographic technologies, resulting in a rapid increase in the aspect ratio of capacitors by 50% or greater, compared to conventional processes. Thus, the collapse of capacitors cannot be prevented by the method mentioned above, or the production cost of the support is sharply increased. There is therefore a need for a method by which the collapse of capacitors can be solved at low production cost.
As mentioned above, the factors that influence the force causing collapse of capacitor patterns include the surface tension (Γ) of washing liquid, the aspect ratio according to the height (H) of storage electrodes, the distance (D) between storage electrodes, the width (W) of storage electrodes, and the contact angle (θ) of washing liquid with regard to storage electrodes.
In a capacitor fabrication process, the wetting step is finished by spin drying the wafer. In this regard, it is important that a washing agent with low surface tension is used to reduce the force when it is evacuated by spin drying. However, currently developed capacitors are too high in aspect ratio to resist the collapse of patterns with a washing agent whose surface tension is only reduced to some degree. At present, there are no existing materials with a surface tension that approximates 0 J/m2, except for liquid helium, and among commercially available surfactants and materials, the lowest surface tension measured is 15 J/m2. A variety of experiments have demonstrated that a washing liquid having such surface tension cannot avoid collapsing capacitors.
In other words, at the present time, the control of the surface tension is not an effective method for preventing the collapse of capacitors.